ES2267607T3 - POWER TRANSMISSION DEVICE FOR WORK MACHINE. - Google Patents

Abstract

A power transmission system for a machine in operation in which the drive power is transmitted from a primary promoter (15, 62) to a drive shaft that holds the work tool (16a, 64) by a reduction mechanism (20, 63) for operating the work tools (17, 65) mounted on the drive shaft in a working state thereof, the reduction mechanism comprising: an input shaft (21, 71); a first gear (42, 73) mounted on the input shaft; an output shaft (30, 75); a second gear (25, 74) mounted on the output shaft and which engages with the first gear a gearbox (33, 76) to accommodate the first and second gears; characterized in that the second gear includes a torque limiter (55, 91) composed of a gear portion (26, 87) and a cylindrical protruding segment (27, 88) adjusted by pressure to the gear portion.

Description

Machine power transmission system working.

This invention relates to power transmission systems for machines in operation such as snow throwers and a manual tractor, and more particularly, to an improvement in the torque limiter of said transmission systems of
power.

A propeller type snow plow is from type in which a spiral blade, known as a propeller, collect the snow and the collected snow is supplied to a blower for download The propeller is driven by a machine or motor electric (hereinafter referred to as promoter in this document primary) through a power transmission system. When the propeller works, the propeller tends to bite a set of ice or stone (hereinafter referred to as materials outside) and the power transmission system suffers a overload, resulting in damage to the system power transmission. To solve this problem, so far it has been proposed to provide a torque limiter in the power transmission system to allow sliding in the power transmission system when subjected to a couple of torsion beyond a given level. One of these examples is described. in the Japanese Public Utility Model Publication No. HEI-2-112715.

The propeller type snow plow includes normally an input shaft driven by the primary promoter and a driving shaft of the propeller to which the power of is transmitted transmission from the primary promoter through a mechanism of power transmission. The torque limiter that forms part of the power transmission mechanism includes a screw Endlessly located in a central portion of the drive shaft of the propeller. The central portion of the drive shaft of the propeller has an outer periphery treated with processes of sulfurization or carburation so that when the axis of Propeller drive is subjected to overload, the screw endless tends to rotate in a slow running state with respect to the axis of propeller drive.

The torque limiter includes the worm screw and propeller drive shaft that is assemble initially accommodating the worm in a box of gears and then adjusting by pressure the shaft of screw propeller drive. This method of assembly is disadvantageous because the worm is retained firmly to hold the propeller drive shaft that is  adjusts by pressing the worm, providing a process of difficult assembly. To solve this problem, practice usual has been to use an improved assembly process in which the worm is initially adjusted by pressure to the axis of propeller drive to provide a structure unitary and later the unitary structure is placed in the box of gears. However, this assembly process requires that the gearbox is divided into two halves on a line perpendicular to the propeller drive shaft, giving as a result a structurally gearbox complicated

The Japanese Utility Model Publication Public No. SHO-58-157029 describes a clutch mechanism applied for use in a manual tractor small size in which the motor output shaft is connected to a drive shaft by means of a clutch. Due to the mechanism of clutch, clutch housing and engine are located at a low level near the ground so that the manual tractor of Small size has a low center of gravity. The mechanism of Clutch also has an overload protection function. Is that is, the clutch mechanism includes a spring, whose force is adjust to a desired value to provide the function of overload protection. A technique that uses a spring to effect overload protection is described in the Publication Japanese Utility Model after review No. SHO-39-4607 titled "Tilling Shaft Load Control Device for Tiller "in which a load control device to prevent bars from breaking of the manual tractor.

In the clutch mechanism of the Publication Japanese Public Utility Model No. SHO-58-157029, the clutch is complicates in structure and therefore a torque limiter Torsion has a complicated structure. The clutch is composed by a clutch release handle and a handle shaft, The clutch mechanism is large. Therefore, all the readiness to accommodate the clutch mechanism is made of greater size in the lateral direction and has an increased weight thus deteriorating the maneuverability of the plow.

Therefore, a first object of the present invention is to provide a power transmission system for a running machine that has a torque limiter torsion that is easy to assemble.

Another object of the present invention is provide a power transmission system for a machine in operation that has a torque limiter that it has a simple, small structure in the lateral direction and of light weight.

In accordance with one aspect of the present invention, a power transmission system is provided for  a machine in operation, in which the power of drive is transmitted from a primary promoter to an axis actuator that holds the work tool by means of a reduction mechanism to operate the operating members mounted on the drive shaft in its state of functioning. The reduction mechanism includes an axis of input, a first gear mounted on the input shaft, a shaft output, a second gear mounted on the output shaft and geared with the first gear, and a gearbox for accommodate the first and second gears. The second gear includes a torque limiter consisting of a portion of gear and a cylindrical protruding segment adjusted by pressure to the gear portion.

The second gear of the reduction mechanism It is part of the torque limiter in which the segment cylindrical protuberant adjusts by pressure to the portion of gear and, the reduction mechanism used by the limiter of the Torque has a simple structure, easy to assemble, which It needs little assembly time and low manufacturing cost. As the torque limiter has a lateral dimension selected to be in the range corresponding to an adjustment by pressing a single stroke of the associated parts, the limiter of the reduced torque and the reduction mechanism  It is small in size and light in weight.

In a preferred embodiment, the mechanism of reduction that is part of the power transmission system it comprises an endless screw reduction mechanism, in which the first gear is a worm gear and the Second gear is a friction wheel.

At least one of the contact surfaces of the protruding segment and the contact surface of the segment of gear can be treated with a sulfurization process so that prevents sliding surfaces from wear and tear prolongs the life of the torque limiter.

Desirably, the input shaft is supported by Rotational way by two bearings, one of which is a angular type bearing. For example, in the case that the axis of input include a bevel gear, the input shaft is subjected not only to thrust load but also to radial load. Using the angular type bearing, both loads are supported by the angular type bearing and the number of bearings

It is preferred that one end of the protruding segment has an angular flange and the gear segment has an annular recess to receive the annular flange of the protruding segment. When the gear segment moves in a thrust direction relative to the gear segment, the gear segment is prevented from moving an excessive amount along the annular flange of the protruding segment. Also, the protruding segment can be reliably positioned with respect to the gear segment during pressing adjustment of the protruding segment to the gear segment by adjusting the annular flange of the flange segment to the annular recess of the gear segment and an assembly process can be simplified. The wheel as a unitary structure. For example, in the case that the second gear is a bevel gear, the placement of the associated parts can be simplified during the pressure adjustment thereof by adjusting the annular flange of the protruding segment to the annular bore of the gear segment, thus allowing an easy assembly of the bevel gear in a structure
unitary.

Certain preferred embodiments of the present invention will be described in detail below, by way of example only, with reference to the attached drawings, in the that:

Fig. 1 is a side view of a machine snow plow that has a power transmission system of according to a first embodiment of the present invention;

Fig. 2 is an exploded perspective view. of the power transmission shown in Fig. 1;

Fig. 3 is a cross-sectional view. taken vertically from the power transmission system of the Fig. 1;

Fig. 4 is a cross-sectional view. taken laterally from the power transmission system of the Fig. 1;

Fig. 5 is a side elevation view of a manual tractor that has a power transmission system of according to a second embodiment of the present invention;

Fig. 6 is an enlarged cross section taken along line 6-6 of Fig. 5;

Fig. 7 is a cross-sectional view. enlarges a piece shown by arrow 7 of Fig. 6;

Fig. 8 is an exploded perspective view of a torque limiter in accordance with the present invention;

Figs. 9A to 9C are illustrative views of the general assembly sequence of the reduction mechanism according to the second embodiment of the present invention; Y

Fig. 10 is a cross-sectional view. enlarged reduction mechanism according to the second embodiment of the present invention.

The following description has a nature simply exemplary and in no way intended to limit the invention, its application or its uses.

Referring now to Figs. 1 to 4, it shows a power transmission system for use in machines in self-propelled operation such as a machine snowplow.

In Fig. 1 the snowplow machine 10 includes a body frame 11 having a pair of tracks laterally spaced 12 (only one shown in Fig. 1) supported by the body frame 11. The handles 13 are connected to a rear portion of the body frame 11 and extend up and back. The ends towards top ends of the handles 13 have handle portions respective 14. A primary promoter, such as an engine 15 is mounted on the body frame 11 to drive the tracks 12. The motor 15 has an output drive shaft 16 that is extends to a front portion of the body frame 11. Drive shaft 16 is connected to a propeller 17 and a a snow blower 18 to transmit drive power to it from the engine 15. Propeller 17 collects snow and snow collection is discharged by the snow blower 18 by means of a trigger 19. Numerical reference 16a indicates an axis of drive that holds the work tool.

A propeller transmission unit 20 of a worm-type reduction mechanism engages through of the drive shaft that holds the work tool 16 with engine 15 to form a power transmission system to transmit the motor drive power to the propeller 17.

Now, the propeller transmission unit 20 that It is part of the power transmission system, it will be described in detail with reference to Figs. 2 to 4

In Fig. 2, the propeller transmission unit 20 is shown as a screw type reduction mechanism without finish. The propeller transmission unit 20 has a screw shaft Endless 21 that serves as input shaft. Screw shaft without end 21 is supported with a pair of bearings 22, 23. A portion Worm shaft front 21 is formed with a worm gear 24, which serves as the first gear. A disk-shaped gear segment 26 has a wheel of external friction 25 serving as a second gear engaged to first gear 24. A cylindrical protruding segment 27 is firmly fits gear segment 26 and has a internal female groove 28. A propeller drive shaft 30 has an external male groove 29 that engages with the female groove 28. The drive shaft of the propeller 30 is rotatably supported with a pair of bearings 31, 32. All the pieces analyzed above are located in a box of changes 34, and the drive shaft of the propeller 30 is supported by gearbox 34 by bearings 31, 32. The gearbox 34 has a front opening that is closed by a lid 35 thus forming a box of gears 33.

The wheel 37 of the type reduction mechanism worm includes two parts, i.e. the segment cylindrical protuberant 27 and gear segment 26. The cylindrical protruding segment 27 is pressure adjusted to gear segment 26, so that the gear segment 26 and the cylindrical protruding segment 27 rotate in a motion unit during normal charging although they freely rotate one with respect to the other during an overload. That is, the segment cylindrical protuberant 27 rotates freely with respect to the segment of gear 26. In this way, a torque limiter is formed torsion. More particularly, the torque limiter is formed by a cylindrical contact surface 38 and a cylindrical contact surface 39 of gear segment 26, with both cylindrical contact surfaces treated with processes of sulfurization, which will be described in detail later. A end of the cylindrical protruding segment 27 has a flange ring 41. The gear segment 26 has a ring gap 42 on which the annular flange 41 of the segment is located cylindrical protuberant 27.

The worm shaft 21 is supported by the bearings 22, 23 the bearing 23 being composed of a angular type bearing.

The sulfurization process is one of the processes of treatment of suitable metal surfaces that disperse free radical sulfide in a surface layer of material iron (such as carbon steel, molded iron, molded steel, stainless steel, etc.) How sulfur free radicals have a high lubricating property, friction is reduced when joined contact surfaces slide together giving as result an increase in ownership anti-wear

When the surface layer of the metal parts, a piece is immersed in a salt bath of a alkali metal salt solution containing sulfur at temperature of 190 ° C and is treated with an anodic oxidation process which forms a dispersed layer of iron sulfide with a thickness of several micrometers To form the iron sulfide layer on the contact surface 38 only the protruding segment cylindrical 27, the cylindrical protruding segment 27 may be masked (i.e. covered with a non-insulating material) in a area except the contact surface. Similarly, the surface of gear contact 42 of gear segment 26 is formed with the iron sulfide layer in the same way analyzed previously.

Now, the assembly process of the propeller transmission 20 will be described in detail by doing reference to Fig. 3.

Initially, the protruding segment cylindrical 27 is adjusted by pressing gear segment 26, unitarily forming with it a wheel 37. The operation of pressure adjustment can be performed in any position arbitrary different from a transmission assembly site of propeller. Particularly, the gear segment 26 is located at first place on a support part of a pressing machine (so that the annular recess 42 is oriented upwards), the cylindrical flange segment 27 provisionally conforms to gear segment 26 from above and force down using a press punch. This downward movement is continue until the annular flange 41 is fully accommodated in the annular recess 42 of the gear segment 26. The annular flange 41 prevents the gear from moving too much in a thrust direction  with respect to the cylindrical protruding segment 27.

As best seen from Fig. 3, a bearing 31 and an oil seal 44 are shown on the box of changes 34 and worm shaft 21 is assembled to the box of 34 changes in a provisional assembly state. Later, wheel 37 is located in gearbox 34. The axle of propeller drive 30 is then inserted transversely to through wheel 37 in a direction shown by the arrow (1) in Fig. 3 so that the male groove of the shaft of propeller drive 30 engages with the female groove 28 of the cylindrical protruding segment 27. Finally, a bearing 32, a retaining ring (C-shaped ring) 45, an oil seal 46 and a cover cap 47 are mounted on the gearbox 34 in a sequential order. Although previously an example of the assembly sequence has been described, the parts essentials of the 20 propeller transmission can be assembled easily in the manner discussed above.

A screw shaft support structure Endless 21 will now be described in detail with reference to the Fig. 4.

Initially, oil seal 48 and the Angular type bearing 23 are mounted on gearbox 34 and the shaft of the worm 21 that carries the sleeve 29 are inserted in an angular type bearing and oil seal 48 in a direction shown by the arrow (2). In one way alternative, sleeve 49, oil seal 48 and bearing angular type 23 are provisionally mounted on the axis of worm 21 and later these parts are assembled in the gearbox 34. The assembled state of these parts corresponds to the provisionally assembled state of the axis of worm 21 shown in Fig. 3.

In a final stage of assembly, the bearing 31 fits cover 35 and a distal end of the screw shaft Endless 21 fits the bearing 31. Subsequently, the cover 35 is joins gearbox 34 and these parts connect to each other by a plurality of bolts 51. In this way, the propeller transmission assembly 20.

When the propeller transmission 20 is assembled, the cover 35 is positioned precisely with respect to the gearbox 34 by at least two positioning organs 52. The box of Changes 34 is filled with lubricating oil 54 through a hole of oil level control of the lid 35 and subsequently the oil level control hole is closed with a plug with bolt shape 53.

The operation of the propeller transmission 20 assembled in this way will now be described in detail by doing reference to Fig. 4.

When the worm shaft 21 serves as the input shaft is rotated by the primary promoter, the worm gear 24 which is geared to the transmission by gears 24 rotates the wheel 37 and the drive shaft of The propeller 30 turns. When this occurs, the worm shaft 21 is subjected to thrust force (thrust load) due to the reaction force in the direction (2). Type bearing angular 23 supports the thrust force. If a bearing is used usual radial, an additional thrust bearing must be provided with a view to withstand the pushing force. As the bearing of angular type 23, however, supports both radial load and load of thrust, the number of bearings may decrease according to the present invention

A torque limiter 55 is formed by means of a union between the cylindrical protuberance 27 and the gear 26 (i.e. contact surfaces 38, 39).

In the event that the propeller drive shaft 30 be subjected to excessive torque beyond a couple of torsion priced when the propeller bites foreign matter, it causes the torque limiter to slide 55 indicated by a continuous circular line. That is, the segment of gear 26 is rotated in a idle state with respect to the cylindrical protruding segment 27. Consequently, it is avoided that the propeller transmission 20 is mechanically damaged, which could occur in a power transmission line.

As previously discussed, such as contact surfaces 38 and 39 of the torque limiter 35 are treated with the sulfurization process, the friction forces that they act on contact surfaces 38 and 39 during the slippage can be reduced and the life of the propeller transmission 20 for a sufficient time higher.

Although, also, the torque limiter 35 is immersed in lubricating oil 54, the heat of friction caused by the sliding of the contact surfaces 38 and 39 is absorbed with lubricating oil 54. Therefore, it is possible avoid an increase in the temperature of the contact surfaces 38 and 39 (i.e. slip surfaces) and the life of The wheel 37 can be greatly extended.

A second preferred embodiment of a system Power transmission for the machine running according to the present invention will now be described by doing reference to Figs. 5 to 10. In the second preferred embodiment, the machine in operation is shown as an example of a tractor manual.

In Fig. 5 the manual tractor 60 includes a body frame 61 mounted thereon, a promoter primary such as a motor 62, a power transmission system mounted on body frame 61 in a position by under engine 62 and including a reduction mechanism 63, a gate drive shaft 64 connected to the reduction mechanism 63, a plurality of tractor bars mounted on the shaft of grating drive 64, a handle 66 extending upwards and backward from a rear portion of the reduction mechanism 63 and a handlebar 67 connected to an upper end of the handle 66. The manual tractor 60 will be referred to as a self-propelled tractor of walking type that is an intermediate case between a tractor self-propeller that is propelled by the output of an engine 62 that serves as a primary promoter, and a traveling type tractor operated by a person. A resistance bar 68 connects to a rear part of the body frame 61. A lever Clutch 69 is supported on an upper end portion of the handlebar 67.

Figs. 6 and 7 show the mechanism of reduction 63 and the bars of the tractor 65 in cross section.

In Fig. 6, the reduction mechanism 63 includes an input shaft 71 that extends downward from a output side of motor 62, a bearing 72 that supports rotary a lower end portion of the input shaft 71, a first bevel gear 73 that forms at a distal end bottom of input shaft 71, a second bevel gear 74 that it fits with the first bevel gear 73 and it has a diameter greater than that of the first bevel gear 73, an output shaft 75 mounted on it the second bevel gear 74, a pair of bearings 75a, 75b, which rotatably support the shaft of exit 75, and a gearbox 76 that accommodates all parts analyzed above. The bearing 72 is a type bearing angular, and bearing 75a is also a type bearing angular.

The gate drive shaft 64 includes a pair of first axes 81, 81 connected to both ends of the axis of output 75, respectively, a couple of second axes 82, 82, connected to the first axes 81, 81, respectively, and a pair of third axes 83, 83, connected to the second axes 82, 82, respectively. The side discs 84 are coupled to ends respective of the third axes 83, 83, respectively.

Fig. 7 illustrates a sectional view enlarged cross section of a section 7 of Fig. 6 related to the reduction mechanism 63.

In Fig. 7, the second bevel gear 74 includes a torque limiter 91 that includes a segment of external gear 87 and a cylindrical protruding segment 88. The internal cylindrical protruding segment 88 is pressure adjusted to the outer gear portion 87.

The torque limiter 91 is formed adjusting the cylindrical protuberant segment 88 by pressure external gear segment 87 under a pressure force default, so when a rated torque is transmits through the torque limiter 91, the segment External gear 87 and cylindrical protruding segment internal 88 rotate in a unit movement and, when the limiter of torque 91 undergoes an overload state, that is, when excessive torque is applied to the torque limiter torsion 91, the transmission connection is released between the segment External gear 87 and cylindrical protruding segment internal 88 and external gear segment 87 rotates in a state of slow march with respect to the cylindrical protruding segment 88. The torque limiter 91 has a lateral dimension L.

The external gear segment 87 has a internal hole 87a having an internal peripheral surface that forms a contact surface 87b. An annular hole 87c is formed at one end of the outer gear portion 87.

The protuberant segment 88 has a portion of shaft 88a and an annular flange 88c formed at a distal end of the shaft portion 88a. The shaft portion 88a has a periphery external cylindrical forming a contact surface 88b. He protruding segment 88 has an internal bore formed by a 88d female groove.

The output shaft 75 has a male groove 75c, which fits with the 88d female groove of the segment protuberant 88 to provide a transmission connection.

The gearbox 76 includes a gearbox changes 92 and a cover 93 that closes a lower opening of the gearbox 92.

Reference numbers 75d and 75e, 75e indicate a retaining ring and oil seals, respectively. A reference number 92a indicates a fill located between the gearbox 92 and the cover
93.

Fig. 8 illustrates a perspective view torque of the torque limiter according to the present invention The torque limiter 91 is formed by the contact surface 88b formed on the portion of gear 87 of the second bevel gear 74, and the surface of contact 88b formed on the outer periphery of the segment protuberant 88. At least one of the contact surfaces 88b of the protruding segment 88 and contact surface 87b of the segment gear 87 can preferably be treated with the process of sulfurization When the gear segment 87 and the protruding segment 88, the drill segment 88 is adjusted by pressure to gear segment 87 in a direction shown by the arrow (3) until the annular flange 88c fits into the gap cancel 87c.

Now, the operation of the second preferred embodiment of the power transmission system of in accordance with the present invention, the Figs. 9 to 10.

Figs. 9a to 9c show a general sequence of reduction mechanism assembly.

Fig. 9A: initially the bearing 75a is fixed to the gearbox 92.

Fig. 9B: then the second one is engaged bevel gear 74 that is preliminary assembled with the first bevel gear 73, moving output shaft 75 in one direction shown by the arrow (4) so that the male groove 75c of the output shaft 75 fits with the 88d female groove of the protruding segment 88.

Fig. 9C: at a later stage, the bearing 75b on the output shaft 75, adjusting the ring of 75d retention in gearbox 92 and mounting the seals of oil 75e, 75e to gearbox 92 from the right sides e left of it. Finally, the filling 92a and the cover 93 are secure to gearbox 92 to close the opening of the same and the lubricating oil is poured into the gearbox 92 from the oil supply port (not shown). Be will therefore understand that, in accordance with the present invention, the essential parts of the reduction mechanism 63 can be assembled in an easy way.

Since the torque limiter has the annular recess 87c and the annular flange 88c, the annular flange 88c can be easily positioned in a fixed location by locating the annular flange 88c in the annular recess 87c and the assembly process can be simplified. In addition, the annular flange 88c prevents the gear segment 87 from moving in a thrust direction (parallel to the output shaft 97) for an amount
excessive

As the bearing 72 of the input shaft 71 is the angular type bearing, the angular type bearing 72 can withstand the radial load and the thrust load acting on the input shaft 71 and therefore the number of bearings

As shown in Fig. 9B, the limiter of the torque 91 is located within the lateral distance L and the lateral dimension of torque limiter 91 is defined as which has a value corresponding to a segment hit protuberant 88 that adjusts by pressure. Consequently, the torque limiter 91 does not have a projected portion excessively laterally and the lateral size of the limiter of the torque 91 can be minimized. Consequently, the mechanism Reduction 63 may have a minimized size.

As shown in Fig. 10, as the limiter of torque 91 is formed by the union between the segment of gear 87 and protruding segment 88 (i.e., between the contact surfaces 87b, 88b), the torque limiter 91 has a simplified structure.

It will now be understood that as the torque limiter torsion 91 is composed of the contact surface 87b formed on the gear segment 87 of the second gear conical 74 and contact surface 88b formed on the segment protuberant 88, the essential parts of the torque limiter torsion 91 are commonly used as the second bevel gear 74 and the torque limiter 91 has a reduced weight.

In case the output shaft 75 is subjected to an excessive load greater than the rated torque, that is, when tractor grills bite incidentally external materials such as stones or pieces of wood, occurs a slip in the torque limiter 91. That is, the gear segment 87 rotates in a idle state with respect to to the protuberant segment 88. As a result, no damage occurs  serious in the power transmission system and also Prevents tractor bars from breaking.

As noted above, at least one of the contact surfaces 87b, 88b, are treated with the sulfurization process Consequently, even when it occurs sliding between contact surfaces 87b, 88b, can prevent the associated parts of the torque limiter 91 have undesirable wear and the life of the second gear conical 74 can extend over a period of time long enough

The primary promoter may include an engine electric. The angular type bearing can be any angular type ball bearing or needle roller bearing angular type

In addition, a connection structure between the shaft propeller drive and protruding segment can be a corrugated connection or key connection.

The machine in operation of the present invention is not limited to the snowplow machine or manual tractor 60, and can be applied to an agricultural machine such as a machine to plant rice or a transplant machine, or a industrial machine.

The torque limiter of the system Power transmission is not limited to gear 25 and gear conical 74.

A transmission system of power for a machine in operation, in which the power transmission is transmitted from a motor (15) to an axis of drive that holds the work tool (16a) by a reduction mechanism (20). The reduction mechanism includes a input shaft (21) having a first gear (24) and a shaft of output (30) having a second gear (25). The second gear has a gear portion (26) and a segment cylindrical protuberant (27) adjusted by pressure thereto, forming the torque limiter (55). As a result, the reduction mechanism, which has the torque limiter, It has a simplified structure, which allows assembly easy.

Claims (5)

1. A power transmission system for a machine in operation in which the power of drive is transmitted from a primary promoter (15, 62) to a drive shaft that holds the work tool (16a, 64) by means of a reduction mechanism (20, 63) to drive the work tools (17, 65) mounted on the shaft drive in a working state thereof, comprising the reduction mechanism: an input shaft (21, 71); a first gear (42, 73) mounted on the input shaft; an output shaft (30, 75); a second gear (25, 74) mounted on the output shaft and that fits with the first gear; a gearbox (33, 76) to accommodate the first and second gears; characterized in that the second gear includes a torque limiter (55, 91) composed of a gear portion (26, 87) and a cylindrical protruding segment (27, 88) pressure adjusted to the portion of
gear. 2. The power transmission system of claim 1, wherein the reduction mechanism (20) is a worm-type reduction mechanism, the first gear (24) is a worm gear and the second Gear (25) is a wheel (37). 3. The power transmission system of claim 1, wherein the protruding segment (27, 88) and the gear portion (26, 87), which form the torque limiter (55, 91), have respective contact surfaces (39, 88b; 38, 87b), at least one of which is treated with a process of
sulfurization 4. The power transmission system of claim 1, further comprising bearings (23, 88) to rotatably support the input shaft (21, 71), when minus one of which is an angular type bearing. 5. The power transmission system of claim 1, wherein the protruding segment has in one of its ends formed with an annular flange (27, 88) and the gear portion (26, 87) is formed with an annular recess (42, 87c) to accommodate the annular flange.